1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one, or several, dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A lithographic apparatus may require a controlled environment, such as a vacuum environment, to be able to transmit radiation from the source to the target portion on the wafer. For example in lithographic apparatus using extreme UV (EUV) radiation (such as 13 nm), a controlled environment is needed, in which specific vapor and gas levels are below a predetermined level. The vapors and gasses may either absorb the EUV radiation, or promote contamination of optical surfaces in combination with the radiation. In an EUV lithographic apparatus, the controlled environment requires that the partial pressure of hydrocarbons (CxHy) is below 10−9 Torr, and the partial pressure of water (H2O) is below 10−7 Torr. Each time the lithographic apparatus is opened, e.g. for maintenance purposes, the controlled environment must be (re-) established. This requires a lot of time (in the order of hours), during which the lithographic apparatus can not be used effectively.
Also, for lithographic apparatus using another type of radiation (e.g. 157 nm or 193 nm), a controlled environment is necessary, although the requirements for the controlled environment may be different from those of EUV type lithographic apparatus. This may require less time to (re-) establish the controlled environment, but still impacts on the effective operational use time of the lithographic apparatus. For example, when using radiation in a range around 157 nm, air and water vapor will absorb the 157 nm radiation. To be able to use the radiation effectively, a controlled environment is required, in which air and water vapor are below a predetermined concentration level. This may be accomplished by purging the beam path of the lithographic apparatus with a purge gas, such as nitrogen (N2), helium (He) or a mixture of gasses which are substantially transparent to the radiation of the projection beam at 157 nm. In 193 nm systems, the radiation is absorbed by atmospheric oxygen, and the controlled environment should substantially eliminate the presence of this gas, e.g. by purging the system with dry nitrogen gas.